Review of the Thermoelectric Properties in Nanostructured Fe₂VAl

Abstract

Besides alloying, nanostructuring was implemented to improve the thermoelectric properties in Fe2VAl. This Heusler alloy indeed displays a thermoelectric figure of merit too small for applications (ZT ~ 0.1 at 300 K) which is caused by a large lattice thermal conductivity (λL = 27 W·m−1·K−1 at 300 K). The effect of nanostructuring on the microstructure and on the thermoelectric properties of alloyed Fe2VAl are therefore reviewed. By mechanical alloying followed by spark plasma sintering, the average grain size (D) was decreased to D ~ 300⁻400 nm in Fe2VAl0.9Si0.1, Fe2VAl0.9Si0.07Sb0.03, Fe2V1.05Al0.95, and Fe2V0.9W0.1Al. As expected, phonon scattering at the numerous grain boundaries lead to a strong decrease in the lattice thermal conductivity, which reached values as small as λL = 3.3 W·m−1·K−1. However, in all the reviewed examples, the thermoelectric figure of merit (ZT) is only marginally or not even improved when comparing to non-nanostructured samples because the electrical resistivity also increases upon nanostructuring. A significantly improved ZT = 0.3 at 500 K was only recently observed in severely deformed Fe2VAl0.95Ta0.05 by high pressure torsion because the very fine microstructure (D ~ 100 nm) strongly enhanced the thermal conductivity reduction.

Keywords

• thermoelectricity
• Heusler alloys
• thermal conductivity
• electrical resistivity
• grain boundaries
• ball-milling
• high pressure torsion